Ultrathin 'Invisibility Cloak' Can Match Any Background

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In the movie "Predator," an alien uses a cloaking device to hide
in plain sight, but the effect is far from perfect: The alien's
attempt to conceal itself is thwarted by distortions of light
bending around it. Now, researchers have built an ultrathin
"invisibility cloak" that gets around this problem, by turning
objects into perfect, flat mirrors.

Invisibility cloaks are designed to bend light around an object,
but materials that do this are typically hard to shape and
only work from narrow angles — if you walk around the cloaked
object, for instance, it's visible. But a new cloak avoids that
problem, and is thin and flexible enough to be wrapped around an
object of any shape, the researchers said. It can also be "tuned"
to match whatever background is behind it — or can even create
illusions of what's there, they added.

Led by Xiang Zhang, director of materials science at Lawrence
Berkeley National Laboratory, the group constructed a thin film
consisting of a 50-nanometer-thick layer of magnesium fluoride
topped by a varying pattern of tiny, brick-shaped gold antennas,
each 30 nanometers thick. (For comparison, an average strand of
human hair is about 100,000 nanometers wide.) The "bricks" were
built in six different sizes, ranging from about 30 to 220
nanometers long and 90 to 175 nanometers wide. [ Now
You See It: 6 Tales of Invisibility in Pop Culture ]

The scientists then wrapped up a tiny, irregularly shaped object
measuring about 36 microns across, or a bit more than
one-thousandth of an inch. Shining a light, with a wavelength of
730 nanometers, or near-infrared, they found that it reflected
back almost perfectly. The
light scattering from the cloak still bounced off the object,
but without revealing where the object was — as though there were
just a flat mirror in its place, the researchers said.

The tiny object appeared to be invisible because the gold
antennas controlled the scattering of the light that reflects off
of it, the scientists explained. Ordinarily, light reflecting off
an object (even a glass mirror) will scatter at least a little,
especially if the shape is irregular. The
waves of light will also sometimes create interference
patterns. As a result, reflected light appears as colors (when
some is absorbed), or reflection, depending on the object.

The new
invisibility cloak changes that: The gold bricks reflect the
light in such a way that the light's phase and frequency are both
preserved. (Phase is an angle measurement that tells you how far
along a light wave you are; two waves 180 degrees out of phase
cancel out.) The ultrathin cloak creates an effect that makes it
seem like the light were hitting a perfect mirror and the cloak
and object weren't even there. Even the edges are invisible with
the new device, the researchers said.

With the proper tuning of the gold bricks, it's not hard to make
the reflected light look like anything you want — either the
background of the object (a floor, for example) or something else
entirely, Zhang told Live Science. If the cloak were big enough,
theoretically, you could drape it over anything. "You could cover
a tank with it and make it look like a bicycle," he added.

The reflection trick also works from any angle, and the
cloak doesn't have to be a certain shape — it can be wrapped
around anything, and the effect still works. It's also thin and
light, according to the researchers.

But there is one disadvantage: If Harry Potter were wearing this
cloak, he'd have to stay still for it to work, since the tuning
has to be matched to the background.

Andrea Alù, an associate professor of electrical engineering at
the University of Texas at Austin, has done extensive research on
cloaking systems. He is skeptical that scientists can
create the kind of illusion Zhang describes.

"They had a small object, a little bump," Alù told Live Science.
"With a larger object, I can't take advantage of that … when I
illuminate it, a portion is not illuminated; it's in shadow." As
such, the illusion of the perfect reflector would be broken, he
said.

Even so, the new findings show you can manipulate how light
reflects using nanometer-scale structures on a thin surface. "The
beauty of the paper is that you can control the reflection
surface at the sub-wavelength scale," Alù said.

Zhang said the cloaking technology's reflectivity offers another
application: displays. Right now, any big projection (e.g., a
movie in a theater) has to use a relatively flat surface. But if
the phase and frequency of the light reflected from it could be
finely controlled, that problem could go away. A projection
surface could be any shape, and the resulting picture would not
be distorted.

Zhang added that this kind of material has been fabricated
before, and that a next step would be to make a lot of it at
industrial scales, tuning the antennas to different wavelengths
of light.